The liquefaction of natural gas is an important and widely-practiced technology to convert the gas to a form which can be transported and stored readily and economically. The energy expended to liquefy the gas must be minimized to yield a cost-effective means of producing and transporting the gas from the gas field to the end user. Process technology which reduces the cost of liquefaction in turn reduces the cost of the gas product to the end user.
Process cycles for the liquefaction of natural gas historically have utilized isentropic expansion valves, or Joule Thomson (J-T) valves, to produce refrigeration required to liquefy the gas. Typical process cycles utilizing expansion valves for ths purpose are described for example in U.S. Pat. Nos. 3,763,658, 4,065,276, 4,404,008, 4,445,916, 4,445,917, and 4,504,296.
The work of expansion which is produced when process fluids flow through such valves is essentially lost. In order to recover at least a portion of the work produced by the expansion of these process fluids, expansion machines such as reciprocating expanders or turboexpanders can be utilized. Shaft work from such expansion machines can be used to generate electric power, to compress or pump other process fluids, or for other purposes. The use of such expanders to expand saturated or subcooled liquid process streams can be beneficial to overall process efficiency under selected conditions. The term "expander" is generally used to describe turborexpanders or reciprocating expanders. In the field of natural gas liquefaction, the term "expander" is usually used to denote a turboexpander, and is so used in the present disclosure.
U.S. Pat. No. 3,205,191 discloses the use of a hydraulic motor comprising a Pelton wheel to expand a subcooled liquefied natural gas stream prior to isentropic expansion through a valve. Conditions are controlled such that no vaporization occurs in the hydraulic motor expander. The expander work can be used for example for driving one or more compressors in the disclosed liquefaction process.
In U.S. Pat. No. 3,400,547, a process is disclosed wherein the refrigeration in liquid nitrogen or liquid air is utilized to liquefy natural gas at a field site for transportation by cryogenic tanker to a delivery site. At the delivery site, the liquefied natural gas is vaporized and the refrigeration so produced is utilized to liquefy nitrogen or air, which is transported by tanker back to the field site where it is vaporized to provide refrigeration to liquefy another tanker load of natural gas. At the field site, subcooled liquefied natural gas is expanded and the expansion work is used to pump liquid nitrogen or air from the tanker. At the delivery site, pressurized liquid nitrogen or air is expanded and the expansion work is used to pump liquefied natural gas from the tanker.
A process to produce liquid air by utilizing refrigeration from the vaporization of liquefied natural gas is disclosed in Japanes Patent Publication No. 54(1976)-86479. In the process, saturated liquid air is expanded in an expansion turbine, and the expansion work is used to compress feed air for initial liquefaction.
U.S. Pat. No. 4,334,902 discloses a process to liquefy a compressed natural gas stream by indirect heat exchange with a vaporizing multicomponent refrigerant in a cryogenic heat exchanger. Precooled two-phase refrigerant is separated into a liquid and a vapor stream; the liquid is further cooled in the cryogenic heat exchanger, expanded in a turboexpander, and introduced into the exchanger where it vaporizes to produce refrigeration; and the vapor stream is further cooled and liquefied in the exchanger, expanded in a turboexpander, and introduced into the exchanger where it vaporizes to produce additional refrigeration. Natural gas at 45 bar is passed through the exchanger, liquefied by indirect heat exchange, and expanded in a turboexpander to about 3 bar to produce liquefied natural gas product. The expansion work of the liquid turboexpanders is used to generate electric power or for other unspecified purposes. Additional refrigeration cycles are disclosed for precooling the refrigerant discussed above, and these cycles also use liquid expanders in which the expansion work is used to generate electric power or for other unspecified purposes.
The use of a turboexpander for the expansion of a liquefied natural gas stream prior to final flash step is disclosed in U.S. Pat. No. 4,456,459. The expansion prior to flash increases the yield of liquefied natural gas product and reduces the amount of flash gas. Work produced by the turboexpander may be usefully employed in the facility to operate various power-driven components through suitable shaft coupled compressors, pumps, or generators.
U.S. Pat. No. 4,778,497 discloses a gas liquefaction process in which a gas is compressed and cooled to produce a cold, high-pressure fluid which is further cooled to produce a cold supercritical fluid. A portion of the cold high-pressure fluid is expanded to provide further cooling and the expansion work is utilized for a portion of the compression work in compressing the gas prior to cooling. The cold supercritical fluid is further cooled and is expanded in an expander without vaporization to yield a final liquid product. A portion of this liquid product is flashed to provide refrigeration for the further cooling of the cold supercritical fluid.
The use of expansion work in a refrigeration or gas liquefaction process to drive pumps or compressors in the same process can improve the efficiency of the process. The optimum integration of expansion work with compression work to yield the greatest overall reduction in capital and operating costs in a given gas liquefaction process depends upon a number of factors. Among these factors are the compositions and thermodynamic properties of the process streams involved as well as mechanical design factors associated with compressors, pumps, expanders, and piping. The present invention as described in the following disclosure allows the improved utilization of expansion work in a process for the liquefaction of natural gas.